1. Field of the Invention
The present invention relates to a mounting member made of glass for a magnetic disk and a method for fabricating the same.
2. Discussion of Background
As shown in FIG. 8, a magnetic disk drive, which has been used as a media unit, secures a plurality of hard disks or magnetic disks 11 between a flange 14 and a clamp 17 by alternately mounting the magnetic disks 11 and spacer rings 10 to a mounting shaft 15 with the flange 14 in stacked fashion, putting a shim 16 on the top magnetic disk 11 and tightening the clamp 17 on the shim by bolts 18. When the magnetic disks are rotated by a rotary shaft 13, magnetic heads 12 read or write information, moving above the magnetic disks in floating fashion.
Each of the magnetic disks has a magnetic film formed on a substrate thereof. As the material for the substrate, there have been known aluminum, glass, ceramics and the like, though only aluminum and glass are put into practical use. As the material for the spacer rings 10, there have been known metal, such as aluminum and stainless steel, glass and ceramics. What is necessary for the magnetic disks is that the distance between a magnetic disk and its related magnetic head becomes as small as possible to record information in high-density and high-capacity. From this viewpoint, the magnetic disks are significantly required to have flatness and surface smoothness. Hard glass with good flatness is extremely superior to an aluminum substrate as the substrate for the magnetic disks since that sort of glass can effectively obtain required surface flatness and is adapted for a reduction in weight and size.
When the magnetic disks 11, the mounting shaft 15, the spacer rings 10, the shim 16 and other mounting members in the magnetic disk drive are different from each other in terms of the thermal expansion coefficient of the materials thereof, a thermal expansion difference is created by a temperature difference between an operating time and a non-operating time, and a magnetic disk 11 is distorted by a strong external force given by its related spacer ring 10. When the magnetic disk 11 is distorted, it becomes difficult to keep the distance between the magnetic disk 11 and its related magnetic head 12 constant all the time during operation. As a result, a change in the distance of the magnetic head 12 to the magnetic disk 11 causes an error in reading or writing information. When the degree of distortion becomes great, there is also a possibility that the magnetic head 12 related to the magnetic disk 11 gets in contact with the surface of the magnetic disk to damage the magnetic film.
In order that, in particular, the magnetic disks 11 and the spacer rings 10 accord with each other in terms of thermal expansion coefficient and minimize the distortion due to a thermal expansion difference so as to avoid a serious problem, it has been proposed that aluminum spacer rings be used for magnetic disks with an aluminum substrate, and that spacer rings made of ceramics having a thermal expansion coefficient approximate to that of glass or made of glass be used for magnetic disks with a glass substrate.
With respect to the fabrication of the spacer ring from glass, i.e., JP-A-10-074350 discloses that a glass ring is first formed, the glass ring has both lateral surfaces as the contacting surfaces for contact with magnetic disks lapped to have required flatness and parallelism, and the glass ring has an electrically conductive film formed thereon such that static electricity charged on a magnetic disk is discharged outside.
Additionally, it has been disclosed in JP-A-9-44969 (corresponding to U.S. Pat. No. 6,215,617) that the material of a holding member, such as a spacer, is selected in accordance with the material of a magnetic disk so as to have thermal expansion coefficient approximate to that of the magnetic disk, e.g., when the magnetic disk is made of glass for instance, ceramics or glass whose thermal expansion coefficient is approximate to that of the magnetic disk is used. It is also disclosed that the contacting surface of a holding member for contact with the magnetic disk is made to have a surface roughness from 0.1 to 0.2 μm in terms of the average roughness Ra at the centerline since rapid rotation causes a slip to occur between the magnetic disk and the holding member when the contacting surface of the holding member is too smooth. It is also disclosed that the holding member has an electrically conductive film coated thereon at a thickness of 0.1 to 3 μm in order that static electricity charged on the magnetic disk is effectively discharged outside.
In conventional magnetic disk drives, glass spacers, whose thermal expansion coefficient is the same as or approximate to that of magnetic disks made of glass, can be used to minimize the distortion of the magnetic disks due to a thermal expansion difference between the magnetic disks and the spacers, avoiding the occurrence of a reading or writing error caused by distortion of a magnetic disk. However, the conventional magnetic disk drives have created a serious problem in that particles (dust) are generated from the glass spacers or the like to disturb the long-term reliability for the magnetic disk drives.
One of the reasons of the dust generation is estimated to be that particles generated during the polishing treatment remain and adhere on the polished surface without being eliminated even after thoroughly washing, and that even if the polished surface is coated with an electrically conductive film as stated earlier, the particles fall away along with parts of the electrically conductive film during a long-term use with deterioration of the electrically conductive film. Another reason is supposed to be that by lapping the contacting surfaces of the spacers for the purpose of improving the flatness and the parallelism of the contacting surfaces and bringing the contacting surfaces into roughened surfaces as stated earlier, the polished surfaces are made of concavities and convexities with relatively sharp leading ends, and that when the magnetic disks are firmly sandwiched between the surfaces with the concavities and convexities, particles come off and drop from sharp ends. In particular, in the case of the spacers being made of ceramics, particles are easily generated from the spacers as porous sintered products in terms of material property, which is notorious in comparison with spacers made of other material.
Since the conventional spacers have the contacting surfaces made of polished surfaces with sharp concavities and convexities, the conventional spacers have created a problem that surface roughness can be increased beyond a certain level as there is a limit to the surface roughness. Only spacers having a small surface roughness and an insufficient anti-slip property have been generally available in practice.
In the case wherein the contacting surfaces of a glass spacer are polished to be brought into roughened surfaces in order to improve an anti-slip property for magnetic disks, when sharp ends of the concavities and convexities forming the polished surfaces come off in use, there is created a problem that the magnetic disks are apt to slip since the clamping force to the magnetic disks mounted to a magnetic disk drive becomes weaker. In order to cope with this problem, there is a proposal to preliminarily make the clamping force stronger. However, when the clamping force is increased beyond a certain level, there is a possibility that the magnetic disks could be distorted.